1. Field of the Invention
This invention relates to a threading apparatus including means for positioning and threading one end of individual pieces of work stock of varying longitudinal dimension. Though not limited thereto, the apparatus is particulary suitable for threading conduit including a protective exterior and interior coating without damaging the intergrity of the coating coextensive with its unthreaded portions.
2. Description of the Prior Art
Various types of threading devices, particularly devices intended for use in applying external threads to a cylindrical conduit, are well known in the prior art. These prior art devices basically comprise two elements: a motor which engages and rotate the conduit, and a cutter or die head which is attached to the motor in engaging relation to the end of the conduit on which threads are to be cut. The operation of such prior art devices may be generally summarized as follows.
Having selected the appropriate die head for the desired threads and conduit size, the operator places the end of the conduit to be threaded through an aperture formed in the motor. Vice-like jaws mounted within the motor in surrounding relation to the aperture are then tightened around the conduit, firmly holding it in place. The motor is then energized causing the jaws and the conduit to rotate rapidly about the conduit's longitudinal axis. It should be noted that the end of the conduit into which threads are to be cut extends approximately six inches from the motor in the direction of the die head. The die head is attached to the motor so that it may be moved in reciprocating fashion along the axis of the rotating conduit.
The die head is then urged into engagement with the rotating conduit, and the dies cut the desired threads. Once the specified section of threads are cut, the motor is switched off, and the dies are opened and released. The threads are then cleaned and finished by a reamer attached to the die head assembly.
With regard to such prior art threading devices, two points are particularly noteworthy. First, by virtue of its attachment to the motor, the die head is limited to a travel distance along the axis of the rotating conduit of approximatey eight inches. Inasmuch as conduit is normally supplied in standard lengths of 10 feet, as much as 9 feet of rotating conduit is often times extending beyond the after end of the motor. Not only does this present a potential hazard, but also this conduit must be supported to prevent it from bending or breaking. Second, severe physical pressure must be applied to the exterior of the conduit by the vice-like jaws of the motor in order to rotate it and prevent it from slipping when engaged by the die head.
While the suitability of such prior art devices can not be disputed in light of their long standing commercial success, recent technological advances in the production of special-purpose conduits have focused the industry's attention on certain inherent shortcomings of such devices. Of particular note is the inability of these prior art devices to thread conduit including a protective exterior and interior coating without damaging the protective coating. Coated conduit is used in application where the conduit will be exposed to a deleterious environment in order to prolong the useful life of the conduit. In such uses it is immediately apparent that any penetration of the protective coating necessarily results in its useful life being shortened. Such is obviously counter productive in economic terms.
Nevertheless, coated conduit must be cut and threaded, and the only means available for theading coated conduit have been those such as described above. In order to hold the coated conduit with sufficient force to accomplish the threading operation, it is necessary to tighten the jaws of the motor sufficiently to penetrate the coating and grip the conduit itself. If the jaws are not tightened sufficiently, the conduit will twist within its coating, resulting in relatively large tears in the protective shield. At the very least, then, it is obvious that the coating is punctured at a plurality of points by the motor's jaws. At the very most, the protective coating is completely torn away.
In recognition of the inherent problems attendant to threading coated conduit, the conduit industry in cooperation with the threading industry has devised a stop-gap solution. Suppliers of coated conduit sell purchasers of coated conduit a coating solution to be used for "touching up" the punctures and tears resulting from the threading operation. Such solution is far from perfect for a number of reasons. First, expensive man hours are consumed in applying the touch-up solution and allowing it to dry. Second, and perhaps even more importantly, such a manually applied coating often time does not provide the quality of protection afforded by the original coating. This, of course, results in more frequent replacement of conduit sections.
Accordingly, it is obvious that there is a great need in the industry for a threading apparatus which is capable of threading coated conduit without thereby damaging the integrity of the coating. Of course, such an apparatus should also be capable of being manufactured at a reasonable price and should be simple to use. Additionally, such a device should be capable of threading uncoated materials, thereby eliminating the necessity of purchasing, maintaining and operating separate threading devices for individual applications.